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BEIRUT — Mortars rained down on the crowded narrow streets of a Palestinian refugee camp in the Syrian capital, killing more than 20 people and injuring dozens as regime forces and rebels clashed on the southern outskirts of Damascus, activists said Friday.

The Britain-based Syria Observatory for Human Rights, which reported the deaths, did not have any details on who had fired the mortars on Yarmouk camp Thursday evening, but Israel Radio reported that the shelling was in fact the work of government troops.

Hamas and the Palestinian Authority condemned the attack.

The state news agency blamed the bombardment on “terrorist mercenaries” — a term the government uses for rebel fighters — and said they had been chased away by security forces.

Government troops, however, have in the past attacked the camp, home to nearly 150,000 Palestinians and their descendants driven from their homes by the war surrounding Israel’s 1948 creation. Palestinians refugees in Syria have tried to stay out of the 17-month old uprising but with Yarmouk nestled among neighborhoods sympathetic to the rebels, its residents were eventually drawn into the fighting.

Yarmouk’s younger inhabitants have also been moved by the Arab Spring’s calls for greater freedoms and have joined protests against President Bashar Assad’s regime— and have died during demonstrations when Syrian troops fired on them.

With the civil war in Syria getting increasingly vicious, chances for a diplomatic solution to the conflict were fading after the resignation Thursday of Kofi Annan, the U.N.-Arab League envoy to Syria. Annan cited divisions within the Security Council preventing a united approach to stop the fighting.

The U.N. General Assembly was preparing to vote Friday on a new Arab-sponsored resolution condemning Syria’s use of heavy weapons to crush the uprising that has killed an estimated 19,000 people since it began on March 11.

The resolution — which like all General Assembly resolution is unenforceable — is expected to denounce Syria for unleashing tanks, artillery, helicopters and warplanes on the people of Aleppo and Damascus, and demand that the Assad regime keep its chemical and biological weapons warehoused and under strict control.

The attack on the Palestinian camp came as clashes raged overnight between rebels and government forces in the nearby Damascus suburb of Tadamon, which was also bombed by army Thursday, sending plumes of black smoke over the city.

The Observatory also reported shelling of the southwestern suburb of Jdaidat Artouz, where dozens of bodies were found after government forces swept through on Wednesday.

Syria’s civil war was largely confined to the cities in the center of the country, like Hama and Homs, as well as the countryside until July, when it came to the capital and northeastern city of Aleppo, Syria’s largest city and main commercial hub.

A rebel assault and revolt in Damascus two weeks ago was vigorously crushed by government forces, but pockets of resistance and sympathetic neighborhoods remain. Sporadic clashes and shelling also continue in Aleppo, especially the rebel bastion of Salaheddine as rebels and government forces hold different parts of that city. On Thursday, the rebels even deployed a captured tank against the regime and briefly shelled an air force base outside Aleppo.

The U.N. peacekeeping chief warned of a major government assault on Aleppo in the coming days to retake the rebel held neighborhoods.

“The focus is now on Aleppo, where there has been a considerable build-up of military means, and where we have reason to believe that the main battle is about to start,” he told reporters in New York late Thursday after briefing the Security Council on his trip to Syria.